Process for electrolytically producing arsenates



Patented Dec. 3, 1940 PROCESS FOR ELECTROLYTICALLY PRODUCING ARSENATESLeo Liiwenstein, Berlin-Wilmersdorf, Germany No Drawing. ApplicationJuly 7, 1937, Serial No. 152,415. In Germany July 11, 1936 1 Claim. (Cl.204-86) Of the arsenates used in practice, particularly in agriculturefor the destruction of pests, fungi and the like, sodium arsenate alonecan be made relatively economically ,by electrolysis. Lead arsenate hasalready been prepared electrolytically, but owing to the powerconsumption and apparatus difficulties the process does not come intoconsideration for technical use.

According to the processes known up to now sodium arsenate is formed byanodic oxidation of sodium arsenite, e. g. on anodes made of nickelsheet. After some time however a layer of solid arsenate forms ontheanode which prevents the further electrolysis. It has already beenproposed to overcome this difliculty by constructing the anode as acylinder from which the salt is removed during continuous rotation.

, This means however that the carrying out of the electrolysis isrendered more difiicult.

It has been found that the electrolysis can be so conducted in thefollowing manner that this obstacle is removed in a technically simpleand economical manner, the energy yield being still further improved.

Operations are carried out with diaphragms in the known manner. Thecatholyte consists of caustic soda solution, the anolyte consists of anaqueous mixture of arsenious acid and caustic soda or sodium nitrite. Incontradistinction to the manner of working heretofore known the anolyteis maintained at an elevated temperature in the anode compartment, whichtemperature amounts to more than C. The solution is continuouslyconducted in a circuit from the anode compartment into a vessel in whichit cools down, when solid sodium arsenate crystallizes out, and is thenled back into the anode compartment again. Care must here be taken thatthe concentrations in the anode compartment remain practically constantby continuous addition of the starting products.

It has been found for example that in the 5 case of an anolytetemperature of -53 C., which has been maintained by heating, bycirculation in the form described a well crystallized sodium arsenatewas obtained on a suction filter which had been inserted into thecircuit. The current, yield was 98%. The tension amounted to 2.5 volts,whilst at ordinary temperature with the usual dimensions and proportionsfor the apparatus and of the electrolyte it was 3.5-3.2 volts. Decreasein the current yield very frequently takes place at raised temperaturein anodic oxidation processes. The good yield obtained was not forthwithto be expected.

What I claim is:

2 The process for preparing sodium arsenate 0 which compriseselectrolyzing as the anolyte in the anode compartment of an electrolyticdiaphragm cell an aqueous mixture of arsenious acid and caustic soda, orsodium arsenite, while 25 LEO LCWENSTEIN. 35

